Rights to copyrighted material must be bought before a screenwriter can adapt any previously published material -- a book, an article, another screenplay, anything.
If you want to write the screenplay to Rainbow Six just for yourself with no intention of ever showing it to anyone in hopes of getting it made, go for it. Worst case scenario, the publisher sees your screenplay on this site and asks you to remove it.
I'm sure you could write one, in the vain of a fan fiction, as a tribute, but as said above you couldn't sell it or anything without getting permission from the person with the rights to it
You'll have to send it to the author before, and only he can give you permission to publish it, but he can also just copy it, because you have no rights at all at that point. It doesnt happen a lot that hollywood or authors steal stories, but they can if they want.
